In the OH-stretching range, most of the micas studied exhibit partial dioctahedral character, which increases with increasing fO2. At both the Ni-NiO and wüstite-magnetite oxygen buffers, our spectra of annite indicate the presence of aluminum and Fe3+ in octahedral sites, which is required to articulate the octahedral layers. The decomposition of the infrared spectra allows one to redefine the assignment of annite absorption bands. Three V-bands of "dioctahedral-type" (in the range 3580–3520 cm-1) are characterized by the bulk number of charges of the two adjacent cations: two bands due to OH groups adjacent to an octahedral vacancy and bonded to Fe2+Al3+square or Fe2+Fe3+square (range 3580–3540 cm-1); a band due to OH groups bonded to Fe3+Fe3+square at 3530 cm-1. For Al-rich annite, the band assigned to OH–Fe2+Fe3+square is replaced by a band due to OH groups adjacent to Fe3+Al3+square. An 8-fold charged environment OH–Fe2+Al3+Al3+ does not seem likely in the micas studied. At higher wavenumbers, two I-bands of "trioctahedral-type" (OH–Fe2+Fe2+Fe3+ and Fe2+Fe2+Al3+) and one N-band corresponding to OH–Fe2+Fe2+Fe2+ have been resolved. For all trioctahedral-type bands (I- and N-bands), a systematic shift is observed toward lower wavenumbers with increasing aluminum content of the mica. When compared to N- or I-bands, V-bands show an opposite behaviour.